const obj = {
  value: 10,
  toString: function () {
    return this.value + 10;
  },
  valueOf: function () {
    return this.value;
  },
};

obj[obj] = obj.value;

console.log("keys:", Object.keys(obj)); // keys: [ '20', 'value', 'toString', 'valueOf' ]
console.log("${obj}:", `${obj}`); // ${obj}: 20
console.log("obj + 1:", obj + 1); // obj + 1: 11
console.log('obj + "":', obj + ""); // obj + "": 10
